An example of a method and an arrangement for detecting an error in the context of measurement quantities in a motor vehicle is disclosed, for example, in German patent publication 4,204,623. The method and arrangement proceed from a measurement value detection system, especially for detecting the position of an actuating element determining the power of an internal combustion engine, such as a throttle flap, wherein the measurement quantity is detected by at least two redundant measuring devices. The measurement signals are supplied to a microcomputer without pull-up or pull-down resistors being present on the signal lines with these pull-up and pull-down resistors having been used up to that time point. For interruptions in the signal line, these resistors serve to pull the signal level at the input of the microcomputer to a defined value which lies outside of the pregiven signal value range. The defective performance of at least one measuring device can be determined by means of a conventional signal range check. Possibly unwanted fault detection could occur for the dimensioning of these resistors which is correct for the purpose intended. To avoid this unwanted fault detection, in the known solution, it has been suggested to do without these resistors for both measuring devices. To detect faults, a controllable electrical switch is inserted in at least one supply line of the measuring devices. This switch is driven in the context of a testing operation to determine interruptions of the signal line and similar fault conditions. However, the complexity of the arrangement is increased with such an electrically actuable switch. Furthermore, the function of the control, which is carried out in dependence upon the measurement quantities, is possibly negatively affected.
It is an object of the invention to provide a method for fault detection in measurement quantities in a motor vehicle which makes possible a reliable fault detection without requiring the additional complexity necessitated in the known solution. Furthermore, it is an object of the invention to provide an arrangement for detecting a fault in measurement quantities in a motor vehicle which makes possible a reliable fault detection while avoiding the above-mentioned disadvantages.
The method of the invention is for detecting a fault in the context of a measurement quantity in a vehicle. The method includes the steps of: providing redundant first and second signal values for the measurement quantity; comparing the time-dependent changes of the two signal values; and, detecting a fault condition when the time-dependent changes do not correlate with each other.
The method of the invention permits a reliable fault detection to be undertaken for measurement quantities in motor vehicles without additional complexity and especially without the complexity associated with switches. It is especially advantageous that the plausibility comparison of two redundant signals of the measurement quantity is undertaken on the basis of the relative signal change and not on the basis of absolute signal values. In this way, lower tolerances can be achieved because the linearity tolerances of the measuring devices are substantially eliminated because of the differential consideration.
With the use of potentiometers as measuring devices, it is especially advantageous that the monitoring can even be carried out when the signals are incorrect because of possible contact resistances in one of the potentiometers. The measuring devices can, for example, be for detecting the position of a power actuating element or an operator-controlled element. This incorrectness does not lead to an unwanted fault detection in the relative plausibilization which is in contrast to the plausibility comparison on the basis of absolute values.
In an especially advantageous manner, pull-up and/or pull-down resistors are omitted in the signal line of the measuring device or very high ohmage resistors are used (much greater than 500 KOhms). The measurement quantities of the measuring devices are evaluated when carrying out a control function. For the redundant measuring device for the above, such resistors are used in a manner known per se in order to reliably detect a defective performance of this measuring device in the context of a signal-range check. This has the advantage that falsifications of the signal of the first measuring device do not lead to an unwanted fault detection in the context of the signal range check because this signal range check is replaced by the differential monitoring of the signal values. Securing this differential plausibilization takes place, inter alia, via the signal-range check in the signal of the second measuring device which reliably detects faults in the region of the signal line and of the second measuring device in a conventional manner because of the pull-up and/or pull-down resistors.